New specification for sandbox interaction and management

[ValentaTomas]

Draft of the improved spec for envd and API.

envd

• REST API server for file upload and download
• (probably) gRPC for interacting with the sandbox

API

• Managing sandboxes and their lifecycle in a more ergonomic way

Tasks:

• [ ] Handle oom_score_adj for processes spawned by envd
• [ ] Handle start command by envd
• [ ] Should we support the client side and bidirectional streaming?
  • WriteFileStream
  • Stdin stream in process handling (ConnectStream, StartStream)
  • Having fewer streams seems like a more stateless option that we should think about.
  • Also streams may be harder for people to use.
• [ ] REST support via gRPC gateway
• [ ] Pass process exit error message
• [ ] Handle auth via gRPC auth
  • Should file upload/download via REST be also behind auth?
• [ ] Watch for new processes?
• [ ] Does ReadFile make sense to return a stream so we can chunk the response? Can this be done by returning only parts of the file instead?
• [ ] Check the HTTP/3 (gRPC)?
• [ ] Add "CreateParents" parameter when uploading a file via REST API